The nature of the thermodynamic cycle on which a heat engine operates requires that heat of combustion be rejected to a waste heat medium. In an air-cooled engine, the medium is air that surrounds the engine. In a liquid-cooled engine, the medium is liquid that circulates through coolant passages in the engine where it is heated before passing to a radiator where the heat is transferred to air that flows through the radiator, although some amount of heat is also rejected directly to air surrounding the engine by radiation and convection.
A motor vehicle typically houses the engine in some sort of a compartment. Most cars and trucks have a front engine compartment that is bounded frontally by a front end structure that includes the radiator and rearwardly by the occupant compartment, or cab. The sides of the engine compartment are bounded by fender structures, and the top by a hood that can be opened to provide access to the engine compartment.
Underhood temperature is a matter of concern to vehicle designers because excessively high temperatures can have adverse effects on the performance and durability of various devices and systems. Space within an engine compartment is often at a premium, and the more crowded an engine compartment becomes, more components are exposed to engine compartment heat, and the movement of air through the engine compartment that can aid to some extent in limiting underhood temperatures becomes more difficult.
Engine operating temperature is affected by various factors. Higher operating temperatures may be necessary in order to enable compliance with relevant emission control regulations. That can add to engine compartment heating.
The cooling system of a liquid cooled engine is typically sized to allow the engine to operate at a desired engine operating temperature, but even when a cooling system is sized to accommodate higher engine operating temperatures, more engine heat is transferred by convention, conduction, and/or radiation to devices in the engine compartment, to the structure bounding the engine compartment, and to air in the engine compartment, and that heat isn't removed by the liquid cooling system. Moreover, placement of a radiator in certain vehicles causes at least some of the engine heat that is rejected at the radiator to pass through the engine compartment.